This invention relates to capacitive switches and more particularly, to radio frequency (RF) microelectromechanical systems (MEMS) capacitive switches and a fabrication process therefore.
RF MEMS capacitive switches have many useful applications for military and commercial RF and microwave applications, RF MEMS capacitive switch comprises a movable metal membrane suspended above a lower electrode and an interposing dielectric layer. An air gap of several microns typically separates the upper membrane from the dielectric layer. The lower electrode comprises a RF signal path, while the upper electrode comprises a RF and DC ground. In the switch “off state”, the air gap between the membrane and lower electrode is sufficient that the upper membrane has an insignificant parasitic capacitance relative to the operating frequency of the switch. When a voltage is applied across the upper and lower electrodes, the electrostatic force pulls the membrane down into contact with the dielectric layer (“on state”). Without a significant air gap, the upper metal membrane, insulator layer, and lower metal electrode form an MIM (metal-insulator-metal) capacitor. This capacitor is designed to achieve sufficient capacitive conductance such that it can capacitively couple, or even short, the RF signal path of the lower electrode to the grounded upper metal membrane. When the applied voltage is released, the restoring force of the membrane metal spring is sufficient to return the membrane to its “off state”.
Electronic switching devices comprise radio frequency microelectromechanical systems (RF MEMS) have many potential benefits over conventional semiconductor devices for controlling and routing microwave and millimeter-wave signals. RF MEMS switches possess very low insertion loss, miniscule power consumption, and ultrahigh linearity. These characteristics make MEMS switches ideal candidates for incorporation into passive circuits, such as phase shifters or tunable filters, for implementation in communications and radar systems at 1 GHz and above.
Despite the excellent RF performance of these devices, their acceptance in industry has been limited by a lack of reliability. In a well-engineered MEMS switch, dielectric charging is the main limitation to lifetime, as opposed to mechanical effects. When the switch actuates, a relatively high voltage (30-50 volts) is applied across a relatively thin switch insulator. The resulting electric field induces charge tunneling into the insulator, where they trapped. As these charges build up, they shift the pull-in and release voltages of the switch. If enough charges become trapped, the operating voltages will shift sufficiently such that the switch will either remain stuck down, or not actuate when desired. In either case, the switch fails to operate properly.
Furthermore, while the RF performance of these devices can be exemplary, reliability issues have limited their deployment into fielded systems, in the case of capacitive MEMS switches, shortcomings relating to dielectric charging have been difficult to mitigate. There are many solutions for lessening the impact of dielectric charging, including hermetic packaging, minimizing the electric field across the dielectric, and tailoring the polarity and waveform of bias control signals to minimize charging. These solutions have provided significant improvements in reliability, but have not proven enough to overcome the “stigma” associated with dielectric charging.
Commercially available RF switches use silicon dioxide (SiO2) or silicon nitride (Si3N4) as a dielectric layer material in a capacitive switch. Charges become trapped in the layer and charge builds over time. As the charge builds, the operation of the device degrades until it fails. In fact, it fails very slowly. Studies have shown that the charge and discharge time constants for these materials are on the order of 10s of seconds to 100s of seconds. After failure, a device may take days to recover because charges trapped in the dielectric layer take so long to recombine at the metal electrodes of the capacitor. The amount of charge accumulated is exponentially related to the applied electric field. The higher the operating voltages, the longer the switches are left in the “on state”. Furthermore, the higher the operating temperature, generally the faster the switch will fail.
More specifically, conventional prior art capacitive switches with oxide or nitride dielectric layers are chosen or designed such that the charges accumulate as slowly as possible. Conventional prior switches slowly degrade until the point of failure. Switches with oxide or nitride dielectrics also possess inherently long discharging time constants. Charging and discharging time constants are approximately equal. Therefore, once failure has occurred, conventional prior art devices are not available for proper operation for a very long time period, rendering the device essentially useless for a majority of applications and uses.
The primary failure mode of conventional prior art RF MEMS capacitive switches is accumulation of electrically charged particles within the insulator layer made of silicon oxide or silicon nitride materials of the switch, in which charges tunnel into and become trapped within the dielectric. The conventional prior art RF MEMS capacitive switch only recovers from this failure after a sufficiently long period of time (hours to days) during which the trapped charges can diffuse or migrate back to the metal electrodes. However, for practical purposes, conventional prior art RF MEMS capacitive switches often fail since the membrane remains stacked to the dielectric layer covering the bottom electrode.
Several techniques have been developed to mitigate the effects of dielectric charging on switch reliability, such as minimizing the operating conditions that lead to dielectric charging, for example, time, voltage, temperature; however, the designer does not often have control over these parameters. Alternatively, design modifications can be made to the switch to enable more reliable operation. One alternative is to minimize the amount of dielectric material within the switch to form a mechanical support of the membrane layer. The dielectric insulating material is patterned into “posts” which support the membrane, but minimize the amount of contact between the dielectric and membrane. Instead of a metal-insulator-metal capacitive switch, it is more properly described as a metal-air-metal switch. This modification trades capacitance ratio (ratio of on capacitance to off-capacitance) for improved reliability.
An alternative method of reducing dielectric charging is to engineer the chemical makeup of the dielectric such that it is conductive, or leaky. Given sufficient conduction within the dielectric, the trapped charges will have more opportunity to recombine in the device current, and thereby be eliminated. However, depending on the physics of the particle charging and discharging, the quiescent current may not always be the proper mechanism for causing the induced charges to dissipate, in which the quiescent current provides no substantial advantage.
There have been attempts to manipulate the bulk conductivity of the dielectric film to bleed off charges and improve reliability. Unfortunately, these techniques have not proven repeatable or sufficient enough to be generally adopted.
Use of diamond or diamond-like carbon (DLC) films as a dielectric for RF MEMS capacitive switches has been suggested. Nanocyrstalline diamond film can be grown using a bias enhanced nucleation (BEN) process at a high temperature (700° C.). Tetrahedral amorphous carbon (ta-C) film can be fabricated for use as a dielectric layer in RF MEMS capacitive switch with improved dielectric properties. However, the diamond-like carbon (DLC) films exhibit high as-grown stress and need to be annealed above 600° C. to release relieve internal stress. Since both BEN grown diamond and DEC films mentioned above involve high temperature processing either during growth or after deposition, they are not compatible for integration with complementary metal-oxide-semiconductors (CMOS) electronics and, therefore, their usefulness is limited, since they cannot be used to fabricate monolithically integrated RF MEMS switches with CMOS devices, which is the ultimate device architecture of interest to manufacturers, who want these integrated devices for insertion into phase array antennas for radars and other RF communication systems. The dielectric properties of ultrananocrystalline diamond (UNCD) films grown at high temperatures have been studied before. However, no reports have been published to date on tuning the dielectric properties of UNCD grown at low temperatures compatible with the CMOS thermal budget, which provides the total amount of energy transferred to a wafer at a given elevated temperation operation (such as ˜400° C.) and their use in RF MEMS switches.
Furthermore, the typical thickness of the dielectric layer is a few 100s of nm and it is challenging to deposit such a thin diamond film without any pin-holes.
It is, therefore, desirable to provide an improved RF MEMS capacitive switch and fabrication process therefore, which overcomes most, if not all of the preceding problems.